Unleaded solder

ABSTRACT

The invention concerns a lead-free solder comprising a bismuth-based alloy. The lead-free solder has a proportion of at least 80 percent by weight of bismuth and a solidus temperature of at least 260° C.

[0001] The invention concerns a lead-free solder.

[0002] Solder is used in a variety of types for the assembly of electronic components, in particular printed-circuit boards. Such electronic components are formed from discrete components in particular resistors, capacitors, diodes and transistors.

[0003] In the course of the miniaturisation of electronic products, the respective electronic circuits are integrated into integrated circuits (IC). Such integrated circuits have a high integration level of electrical circuits and connections. Furthermore, the individual integrated circuits are applied to printed-circuit boards or similar in a high packing density whereby the printed-circuit board itself comprises a large number of conductor layers.

[0004] To solder integrated circuits onto such printed-circuit boards, only an extremely small space remains within which the solder has to be exactly positioned.

[0005] In doing so, considerable mechanical demands are also put on the solder. In particular, the solder must be resistant to shear and compressive forces. In addition, the solder must demonstrate a good thermal fatigue strength, that means the solder must be insensitive to volumetric changes with large temperature fluctuations.

[0006] These requirements are fulfilled by solder that contains lead as the principle component. Alloys of the type PbSn(x)Ag(y) are especially used for the electrical bonding of electronic components on printed-circuit boards. Solder of this type has a solidus temperature that lies above 260° C. Solder of this type is therefore also particularly suited for the bonding of power semiconductors by the operation of which a considerable heat generation develops.

[0007] An important disadvantage of solder of this type exists in that the lead in this solder has strongly toxic characteristics and therefore presents a danger to human health. Therefore, strict safety measures have to be met for the disposal of components with solder containing lead. The trend in the legislation goes towards setting high requirements for the disposal of materials of this kind.

[0008] Attempts have therefore been made to replace those types of solder containing lead with lead-free solder.

[0009] Lead-free solder of this type is typically formed from tin-based alloys.

[0010] The problem here is however that such types of tin-based solder demonstrate a low solidus temperature.

[0011] Furthermore, such types of solder have unsatisfactory mechanical characteristics. In particular such types of solder have insufficient ductility, strength and durability with thermal loads.

[0012] In DE 195 38 992 A1, a lead-free solder is suggested that is improved with respect to these mechanical characteristics. This solder consists of an alloy that has effective amounts of bismuth and tin as well as gold. At the same time, the alloy has a melting temperature of around 138° C. The alloy preferably consists of a percentage by mass of 42% tin, a percentage by mass of less than 1% gold and a percentage by mass of around 58% bismuth.

[0013] The disadvantage with this solder is however that, because of the low melting temperature, its use for the bonding of electronic circuits and, in particular, power semiconductors is not possible or only extremely limited. Especially the strong heat generation with power semiconductors prevents the use of such types of solder.

[0014] From WO 9747425, a lead-free solder is known that consists of an alloy of tin, bismuth and silver. The tin proportion of the alloy lies between 70% and 93%. Furthermore, the solder contains 5% to 27.5% bismuth and 2% to 7.5% silver. The alloy available in powder form is particularly used for the manufacture of solder pastes.

[0015] In EP 826458, a lead-free solder based on tin is described. As an additional component, the solder contains silver with a proportion of 3% to 4%, bismuth with a proportion of 2% to 6% and indium with a proportion of 2% to 6%. The solidus temperature of this solder alloy lies at a maximum of 211° C.

[0016] The object of the invention is to provide a lead-free solder with high thermal and mechanical capacity.

[0017] The features of claim 1 are foreseen for the solution to this task. Applications of the solder in accordance with the invention are described in claims 14 and 15. Advantageous application types and purposeful further developments of the invention are described in the sub-claims.

[0018] The lead-free solder in accordance with the invention consists of an alloy on the basis of bismuth with a proportion of at least 80 percent by weight bismuth and with a solidus temperature of at least 260° C.

[0019] As the solder in accordance with the invention contains no proportions of lead, it is extremely environmentally compatible.

[0020] At the same time, the solder in accordance with the invention has a high solidus temperature which makes it possible to use the solder for the bonding of integrated circuits.

[0021] The solidus temperature of at least 260° C. lies in a range in which normally only solidus temperatures of solder containing lead lie. The liquidus temperature of the solder in accordance with the invention lies preferably below 450° C. Based on these thermal characteristics, the solder in accordance with the invention can be used in an extremely wide range for the bonding of electronic components, in particular highly integrated circuits.

[0022] The solder in accordance with the invention is particularly suited for the bonding of power semiconductors by the operation of which considerable quantities of heat are released.

[0023] The solder in accordance with the invention can be used particularly for modern soldering techniques that are used for the bonding of highly integrated circuits. This especially includes flip-chip connecting techniques and so-called die-attach techniques with which chips and things of that kind are applied to a substrate for mechanical bonding by means of the solder.

[0024] With soldering techniques of this type, increased demands are placed on the solder with a view to its thermal and mechanical characteristics.

[0025] This particularly includes the thermal fatigue strength of the solder. The greater the fatigue strength of the solder, the greater its resistance to mechanical deformations which are caused by large temperature fluctuations.

[0026] A further important parameter is the wettability of the solder. The better the wettability of the solder, the better it can be applied to the surface of a substrate. This parameter is particularly significant for the electrical bonding of highly integrated circuits as in applications of this type extremely small amounts of solder have to be applied with positioning accuracy onto the substrate. In doing so, it must be guaranteed that these small amounts of solder demonstrate a high retention strength on the substrate. For this, in particular a high wettability of the solder is of importance.

[0027] Further mechanical parameters of the solder are its shear strength and flexural strength. This means that with force influences on electronic components secured to a substrate by means of the solder, the solder must not be damaged by lateral forces or shear forces. The strength of the solder ultimately forms an essential mechanical parameter.

[0028] With a view to these thermal and mechanical characteristics, the solder in accordance with the invention demonstrates good values so that the solder has a high resistance to thermal and mechanical loads.

[0029] The thermal and mechanical characteristics can be flexibly and reproducibly given by the selection of further solder components.

[0030] In particular, in an advantageous application type of the invention, the thermal and mechanical characteristics can be altered by the addition of silver.

[0031] In doing so, it has been proved that with increasing silver content the mechanical characteristics of the solder in accordance with the invention are improved. In particular, by increasing the silver content the wettability of the solder is increased. The ductility of the solder is also increased with the addition of silver.

[0032] The solder in accordance with the invention is especially used for the manufacture of semi-finished products such as pastes, wires and tapes for example.

[0033] Because of the good ductility of the solder in accordance with the invention, in particular with alloys containing silver on the basis of bismuth, these semi-finished products can be easily manufactured with high quality. In particular, a good mechanical workability of such semi-finished products is guaranteed. For example, wires that are manufactured with the solder in accordance with the invention, obtain a good spooling ability. Furthermore, because of the high breaking elongation of the solder in accordance with the invention, a good handling of the semi-finished products, in particular the wires and tapes, is obtained.

[0034] In the following, the invention is explained based on embodiments and a drawing. The only FIG. 1 shows a schematic presentation of a semiconductor chip that is bonded to a substrate by means of the solder in accordance with the invention.

[0035]FIG. 1 shows schematically a semiconductor chip 1 that is soldered to a substrate 2, preferably a substrate made of metal, a so-called leadframe, or to a ceramic substrate.

[0036] Copper is often used as material for the leadframe. The metallic substrate can, but does not have to be, coated with an additional metallic layer 3. With a ceramic substrate on the other hand, a metallic layer 3 is necessary. This preferably consists of copper, nickel, silver or gold.

[0037] A single-layer or multi-layer printed-circuit board with one or several printed conductor layers can also serve as the substrate. It is important that the printed-circuit board can be heated to at least the solidus temperature of the lead-free solder that, with the solder in accordance with the invention, amounts to at least 260° C.

[0038] The back of the semiconductor chip 1 is also coated with a metallic layer 4 or with a series of several layers. The materials gold, silver, nickel and titanium are used for this whereby the outermost layer generally consists of gold.

[0039] A solder layer 5 formed from the lead-free solder in accordance with the invention is foreseen for the mechanical and thermal bonding of the semiconductor chip 1 to the substrate 2. The semiconductor chip 1 is attached to the substrate 2 or to the layer 3 of the substrate by means of the solder layer 5.

[0040] Application of the solder in accordance with the invention is not restricted to the arrangement according to FIG. 1. The solder in accordance with the invention is generally suited for the bonding of electrical and electronic components, in particular also discrete components such as diodes, transistors, resistors and things of that kind.

[0041] The solder in accordance with the invention is used with particular advantage for the bonding of power semiconductors by the operation of which a high heat generation occurs. Furthermore, the solder in accordance with the invention is used for the bonding of highly integrated circuits. In doing so, the bonding of circuits of this type takes place on substrates with, for example, flip-chip techniques, die-attach techniques or similar.

[0042] The lead-free solder in accordance with the invention consists of an alloy on the basis of bismuth whereby the proportion of bismuth amounts to at least 80 percent by weight.

[0043] The solder in accordance with the invention has a solidus temperature of at least 260° C. and preferably a liquidus temperature that lies below 450° C.

[0044] Because of these thermal characteristics, components such as power semiconductors with which large amounts of heat are released can also be bonded with the solder in accordance with the invention. At the same time, the solder with high long-term stability can also be used for arrangements of components of high package density. In particular, integrated circuits with a high number of electrical connections can also be safely bonded.

[0045] Furthermore, the solder in accordance with the invention has good mechanical characteristics. In particular, the alloy on a bismuth basis has a high strength.

[0046] The solder in accordance with the invention also has good characteristics with a view to the thermal fatigue strength. Furthermore, the solder in accordance with the invention has a good wettability in particular on substrate layers 3 in accordance with the embodiment in accordance with FIG. 1.

[0047] Apart from bismuth as the principle component, the solder in accordance with the invention also has silver. Preferably, the silver proportion of the alloy amounts to between 0.5 and 15 percent by weight.

[0048] By varying the proportion of silver, the mechanical characteristics of the solder in accordance with the invention can be selectively changed.

[0049] In particular, the ductility of the solder increases with increasing silver content. At the same time, the elasticity of the solder also increases by means of which ultimately the shear strength, the flexural strength and the breaking elongation of the solder are also increased.

[0050] A significant improvement in the named mechanical characteristics is obtained with a proportion of at least 1.5 percent by weight of silver.

[0051] To optimise the characteristics of the solder in accordance with the invention, in particular the mechanical parameters, further components are added to the alloy on the basis of bismuth in addition to or as an alternative to the named silver proportion.

[0052] A first component of this type is formed by copper which is preferably contained in the solder in accordance with the invention with a proportion of 5 percent by weight.

[0053] Tin can also be contained as a component in the solder in accordance with the invention.

[0054] In contrast to known lead-free solder, tin does not form the main component of the alloy but is limited to a proportion of maximum 3%. By limiting the tin proportion in the alloy, it is guaranteed that the solidus temperature of the solder lies above 260° C.

[0055] As further components, antimony and/or zinc and/or indium and/or magnesium are contained in the solder. The proportion of these components is limited respectively to a maximum of 3 percent by weight.

[0056] As further components of the solder, arsenic and/or phosphorus can be foreseen. Their proportion is limited respectively to a maximum of 1 percent by weight.

[0057] The solder in accordance with the invention is used for the manufacture of semi-finished products such as pastes, wires and tapes for example.

[0058] An important advantage of the solder in accordance with the invention, in particular in application types that, apart from bismuth as the main component also contain significant proportions of silver, is the good workability.

[0059] This workability is a significant prerequisite for the manufacture of semi-finished products.

[0060] For the manufacture of solder pastes, powders comprising the solder are necessary that have defined particle sizes and which are charged with oxygen to the smallest possible extent. The solder in accordance with the invention is also well suited for this.

[0061] For the manufacture of tapes, the proportion of the individual components of the alloy on the basis of bismuth are selected so that simultaneously with good workability a high strength of the solder is obtained. At the same time, the tapes made of the solder in accordance with the invention have a good mechanical load capability. In particular, the tapes can be well rolled and bent without fractures occurring in the tapes.

[0062] For the manufacture of wires, the proportions of the individual components of the solder are selected so that the wires can be easily rolled.

[0063] Generally, by means of the suitable selection of components of the solder, a good handling of the semi-finished products manufactured with it is achieved. 

1. Lead-free solder consisting of an alloy on the basis of bismuth with a proportion of at least 80 percent by weight of bismuth and with a solidus temperature of at least 260° C.
 2. Solder according to claim 1, characterised in that this contains silver as a further component.
 3. Solder according to claim 2, characterised in that this contains a proportion of 0.5 to 15 percent by weight of silver.
 4. Solder according to claim 2 or 3, characterised in that the proportion of silver amounts to at least 1.5 percent by weight.
 5. Solder according to one of claims 1 to 4, characterised in that this contains copper as a further component with a proportion of up to 5 percent by weight.
 6. Solder according to one of claims 1 to 5, characterised in that this contains antimony as a further component with a proportion of up to 3 percent by weight.
 7. Solder according to one of claims 1 to 6, characterised in that this contains zinc as a further component with a proportion of up to 3 percent by weight.
 8. Solder according to one of claims 1 to 7, characterised in that this contains indium as a further component with a proportion of up to 3 percent by weight.
 9. Solder according to one of claims 1 to 8, characterised in that this contains magnesium as a further component with a proportion of up to 3 percent by weight.
 10. Solder according to one of claims 1 to 9, characterised in that this contains arsenic as a further component with a proportion of up to 1 percent by weight.
 11. Solder according to one of claims 1 to 10, characterised in that this contains phosphorus as a further component with a proportion of up to 1 percent by weight.
 12. Solder according to one of claims 1 to 11, characterised in that this contains tin as a further component with a proportion of up to 3 percent by weight.
 13. Solder according to one of claims 1 to 12, characterised in that its liquidus temperature lies below 450° C.
 14. Use of the solder according to one of claims 1 to 13 for the manufacture of semi-finished products, in particular wires, pastes and tapes.
 15. Use of the solder according to one of claims 1 to 14 for the soldering of electronic components to a substrate (2).
 16. Use of the solder according to claim 15, characterised in that the electronic components are formed from integrated circuits and/or power semiconductors.
 17. Use of the solder according to one of claims 15 or 16, characterised in that the substrate is coated with a layer (3) made of copper, nickel, silver or gold. 